The present invention relates to methods for manufacturing a carburized part, and steel parts.
Conventionally, for mechanical parts such as gears, which are required to have high surface hardness, JIS (Japanese Industrial Standards) steels such as SCR420 are used by processing the steel into a shape of a part, and then performing surface hardening through case hardening.
At this time, the surface hardness and the strength can be increased if fine carbides can be produced on the surface, and also a large amount of carbides can be produced by the case hardening.
Actually, however, conventional case hardening methods cannot produce a large amount of such fine carbides on the surface in a desirable manner, and cannot increase the surface strength sufficiently.
Conventionally, a method for performing induction hardening after carburizing is known as a case hardening method. This method can be said to be an excellent method capable of providing both advantages of carburizing and advantages of induction hardening.
In the case where case hardening is performed by a common method in which direct quenching is performed after carburizing, martensite is formed up to the inside of the part, whereby the residual strain is increased. However, the strain is small in the case where induction hardening is performed after carburizing. This advantage is obtained because the induction hardening is capable of hardening only the surface without changing the inner structure of the part.
However, SCR420, which is a steel for carburizing, has a high content of Cr that tends to form carbides. Thus, when SCR420 is used, carbides tend to be produced during the carburizing process, and these carbides are less likely to melt in the subsequent induction hardening process. Therefore, it is difficult to obtain a carburized part with a uniform surface structure.
Moreover, fracture, starting from the residual carbides on the surface, tends to occur, whereby the strength is reduced.
In conventional common gas carburizing, the hardness has a highest value at a surface carbon concentration of about 0.8%, and it is difficult to increase the surface carbon concentration to a value higher than about 0.8%.
On the other hand, the surface carbon concentration can be increased to a value higher than about 0.8% in the case where vacuum carburizing is performed as a carburizing technique.
However, since SCR420 has a high content of Cr that tends to form carbides, increasing the surface carbon concentration by vacuum carburizing tends to further produce Cr carbides, and these Cr carbides become coarse. Such Cr carbides do not melt in the subsequent induction hardening process, whereby fracture, starting from the carbides, is further facilitated, reducing the strength.
In view of the above problems, the surface carbon concentration in carburizing is conventionally reduced to prevent production of carbides. In this case, however, the surface hardness is insufficient, and thus, a sufficient strength is not obtained.
Note that a carburized part using a steel having a composition similar to that of the present invention is disclosed in Japanese Patent Application Publication No. J-PA-2007-291486 as a related art of the present invention.
However, the carburized part disclosed in Japanese Patent Application Publication No. JP-A-2007-291486 is completely different from the present invention in a hardening technique following a carburizing process, and in a technique of producing carbides on the surface of the carburized part. Thus, the carburized part of Japanese Patent Application Publication No. JP-A-2007-291486 is different from the present invention.
Moreover, Japanese Patent Application Publication No. JP-A-2008-63603 shown below describes that an induction hardening step is performed after a carbonitriding step in a manufacturing method of a track member. However, the method disclosed in Japanese Patent Application Publication No. JP-A-2008-63603 is completely different from the present invention in the composition of a steel material, and thus, is also different from the present invention in a technique of producing carbides. Thus, the method in Japanese Patent Application Publication No. JP-A-2008-63603 is different from the present invention.